1. Field of Invention
The invention relates to power supplies, and more particularly to the structures and control method of uninterruptible power supplies.
2. Related Art
Uninterruptible power supply systems have been widely used to protect electrical equipments from damages or failures caused by utility power disturbances. There are ten commonly recognized utility power disturbances; nine of the disturbances are voltage related and one of the disturbances is frequently related.
International standard IEC62040-3 introduces a three step UPS classification code to standardize the UPS performance, which is briefly illustrated in “A New International UPS Classification by IEC 62040-3” (The 24th International Telecommunication Energy Conference, 2002.) by Wilhelm Solter According to the definition of IEC62040-3 STEP1, VFI represents that UPS output voltage is independent of the input supply voltage and frequency variations. VI represents that UPS output voltage is dependent on the input supply frequency variations, but is independent of the supply voltage variations. VFD represents that UPS output is dependent on the input supply voltage and frequency variations. IEC62040-3 STEP1 defines VFI as UPS classification 1, VI as UPS classification 2 and VFD as UPS classification 3. The double conversion UPS is the only structure to meet the VFI requirement.
However, though most double conversion UPS feature the VFI, the high power loss and poor efficiency resulted from their intrinsic series-series structures have inspired the invention of new UPS structure; for example, an UPS structure according to U.S. Pat. No. 5,686,768, an UPS structure according to U.S. Pat. No. 6,744,648 and in FIG. 1, a structure also known as the Delta conversion UPS.
All these structures can be regarded as a series-parallel structure which inherently generates less power losses and hence higher efficiency as it transmits real power from an AC power source to load only through one conversion. As shown in FIG. 1, this series-parallel structure uses a series converter 503 coupled between the AC power source 501 and load 502 of UPS, and an inverter 504 coupled between the DC power source 505 and load 502 of UPS. The series converter 503 is controlled to draw real power from input AC power source 501 and deliver it to the load 502; the inverter 504 generates a regulated voltage to the load 502. By the structure and control methods, the series converter 503 is able to compensate the voltage difference between the input and output voltage generated by the inverter 504, and draw a high power factor current from the AC power source 501; the inverter 504 is able to compensate the reactive power drawn by the load 502 and provide a regulated voltage to the load 502.
However, because the power of the AC power source is directly coupled to the load, the series-parallel UPS can function properly to receive power from the AC power source only when the frequency and phase between the AC power source and load of the UPS are tightly synchronized so that the converter and the inverter deliver in phase current to the load. If they are not synchronized, the inverter may deliver current while the converter draws current. This causes an overcurrent condition in the inverter or the shutdown of the converter. Because frequency regulation of the input AC power source is not provided, the series-parallel UPS falls in the VI (voltage independent) classification defined by IEC62040-3 STEP1. The series-parallel UPS is inferior to the double conversion UPS.
In addition to the VI characteristic, the prior art series-parallel structures have other disadvantages; the Delta conversion UPS needs a bulky and expensive line frequency (60/50 HZ) transformer, shown as T1 in the FIG. 1; the structure of U.S. Pat. No. 5,687,768 needs no line frequency transformer but, in buck mode operations as in the FIG. 6 and the FIG. 8 of the specification of the patent, it generates discontinuous or pulsating current to the AC voltage source (utility power) causing severe electromagnetic interference (EMI) problems; the structure of U.S. Pat. No. 6,744,648 doesn't draw pulsating current from the AC input power source but it also needs a transformer to avoid the buck mode operation. And still, one common disadvantage of the prior art series-parallel structure is in the normal mode operation when supplying power to a nonlinear load. A nonlinear load generates significant reactive power that is assumed by the inverter of the UPS of a series-parallel structure; this reactive power contributes to additional power loss and deteriorates the efficiency of the UPS compared with the same UPS with a linear load generating no reactive power. All these disadvantages make the series-parallel structure less appealing compared with the double conversion structure.
Nevertheless, IEC62040-3 is not all favorable to the double conversion structure. It is common that loads in a UPS draw high start up current when the loads are applied to the UPS. The start up current can several times higher than a rated current to be normally protected so that the output of UPS is compromised unless the UPS is designed with several times the overload capacity IEC62040-3 STEP3 defines three classes for the output voltage tolerances of UPS upon a linear and nonlinear step load. In terms of IEC62040-3 STEP3, the series-parallel UPS is better than the double conversion UPS in providing high quality output voltage because the converter and the inverter of the series-parallel UPS jointly provide power to the load. By comparison, the power to the load is solely provided by the inverter in the double conversion UPS.
Unless enhancing the overload capacity of the inverter circuit, the double conversion UPS will be inferior to the series-parallel UPS per IEC62040-3 STEP3. An example for illustration is shown in U.S. Pat. No. 6,160,722, wherein the UPS structures according to the invention has an AC power source and a DC power source to supply the DC bus for the inverter to supply power to the load. Although the UPS has two power sources supplying power simultaneously, the power to the load is supplied solely by the inverter. Thus, the inverter itself became the bottleneck for supplying power to the load.